Gypsum composite modifiers

ABSTRACT

Anhydrite calcium sulfate whiskers are prepared by combining alpha calcium sulfate hemihydrate and water to form a slurry, autoclaving the slurry to form alpha calcium sulfate hemihydrate whiskers in water, dewatering the alpha calcium sulfate hemihydrate whiskers, and heating the alpha calcium sulfate hemihydrate whiskers to form anhydrite calcium sulfate whiskers. A composite includes alpha-derived anhydrite calcium sulfate whiskers and a base material and is prepared by combining alpha-derived anhydrite calcium sulfate whiskers with a base material to form a composite.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Applications No.61/912,609, filed Dec. 6, 2013, and 62/074,163, filed Nov. 3, 2014, thedisclosures of which are each incorporated by reference herein.

BACKGROUND

The present disclosure relates generally to the field of calciumsulfate-based products and manufacturing, and more particularly tocalcium sulfate whiskers and composites and methods for their use andmanufacturing.

Various fillers are known for use as reinforcement, opacification and/oradditives in paint, coatings, papermaking, and plastics. For example,fillers such as fiberglass, carbon black, calcium carbonate, silica,talc, kaolin, and aluminum hydroxide are currently used in theseapplications. However, fibrous fillers generally provide improvedstiffness, strength, and thermal stability properties to composites, ascompared to particulate and platelet shaped fillers.

Single crystal fibers, also known as “whiskers,” have been shown toprovide improved properties in composites as compared to traditionalfibers. For example, whiskers may provide improved surface quality andaesthetics for composites, because they are typically finer and smootherthan fibers. Whiskers may also provide improved dimensional and thermalstability, increased strength and toughness, and higher fluidity (e.g.,for improved mold casting). However, whiskers are difficult toeconomically manufacture, due to the slow growth rate of whiskers.

Accordingly, there is a need for improved whiskers and composites madetherewith, as well as for methods for use of and manufacturing improvedwhiskers.

SUMMARY

In one aspect, a method of making anhydrite calcium sulfate whiskers isprovided, including: (i) combining alpha calcium sulfate hemihydrate andwater to form a slurry; (ii) autoclaving the slurry to form alphacalcium sulfate hemihydrate whiskers in water; (iii) dewatering thealpha calcium sulfate hemihydrate whiskers; and (iv) heating the alphacalcium sulfate hemihydrate whiskers to form anhydrite calcium sulfatewhiskers. Anhydrite calcium sulfate whiskers made by this method arealso provided.

In another aspect, a method of making a composite is provided, includingcombining alpha-derived anhydrite calcium sulfate whiskers with a basematerial to form a composite.

In yet another aspect, a composite is provided, including alpha-derivedanhydrite calcium sulfate whiskers and a base material.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, which are meant to be exemplary and notlimiting, and wherein like elements are numbered alike:

FIG. 1 is a micrograph showing alpha-derived anhydrite calcium sulfatewhiskers prepared in accordance with one embodiment of the methodsdisclosed herein.

FIG. 2 is a micrograph showing beta-derived anhydrite calcium sulfatewhiskers.

FIG. 3 is a graph showing the elastic modulus of composite samplesprepared in accordance with one embodiment of the methods disclosedherein, as tested according to three measuring methods.

DETAILED DESCRIPTION

Disclosed herein are calcium sulfate-based whiskers and composites andmethods for their use and manufacturing. These whiskers, composites, andmethods meet one or more of the above-described needs by providingsingle crystal fibers and composites made therewith, having improvedproperties. For example, the whiskers may have an improved aspect ratiocompared to known fibers and/or a faster growth rate.

Whiskers and Methods of Making

In certain embodiments, whiskers are provided. As used herein, the term“whiskers” refers to single crystal fibers. Generally, the whiskersdisclosed herein are made using calcium sulfate, which may also bereferred to as gypsum or plaster. Calcium sulfate may exist as ahemihydrate (CaSO₄*1/2H₂O), a dihydrate (CaSO₄*2H₂O), or an anhydrite(CaSO₄). Generally, the beta calcium sulfate hemihydrate, which isobtained from the calcination of dihydrate calcium sulfate under anelevated temperature at ambient temperature, may be used to preparecalcium sulfate whiskers. However, it has been discovered that alphacalcium sulfate hemihydrate provides surprising advantages to whiskerprocessing and properties. Alpha calcium sulfate hemihydrate, which hasthe same chemical composition as the beta form, has gone through apressurized calcination process, which produces well-crystallized,prismatic particles. For example, the pressurized calcination processmay be an autoclave process. It should be understood that while thepresent disclosure generally relates to alpha calcium sulfate whiskers,the use of other suitable materials is also envisioned and intended tofall within the scope of this disclosure. For example, other particlesthat have been subjected to a pressurized hydrothermal reaction processmay also be used.

In certain embodiments, methods of making anhydrite calcium sulfatewhiskers include: (i) combining alpha calcium sulfate hemihydrate andwater to form a slurry; (ii) autoclaving the slurry to form alphacalcium sulfate hemihydrate whiskers in water; (iii) dewatering thealpha calcium sulfate hemihydrate whiskers; and (iv) heating the alphacalcium sulfate hemihydrate whiskers to form anhydrite calcium sulfatewhiskers.

In one embodiment, the step of combining the alpha calcium sulfatehemihydrate and water includes combining the alpha calcium sulfatehemihydrate with the water such that the alpha calcium sulfate ispresent in the slurry in an amount from about 1 to about 30 percent byweight of the slurry. For example, the alpha calcium sulfate hemihydratemay be combined with the water such that the alpha calcium sulfate ispresent in the slurry in an amount from about 1 to about 20 percent byweight of the slurry, from about 2 to about 15 percent by weight of theslurry, or from about 2 to about 10 percent by weight of the slurry. Inone embodiment, the alpha calcium sulfate hemihydrate may be combinedwith the water such that the alpha calcium sulfate is present in theslurry in an amount from about 5 to about 8 percent by weight of theslurry. Without being bound by a particular theory, it is believed thatthe combination of alpha calcium sulfate hemihydrate and water resultsin dihydrate crystals in the slurry.

In certain embodiments, the alpha calcium sulfate hemihydrate has amedian particle size from about 1 micron to about 100 micron, such asfrom about 1 micron to about 20 micron. For example, the alpha calciumsulfate hemihydrate may have a median particle size from about 2 toabout 5 micron. In certain embodiments, methods of making anhydritecalcium sulfate whiskers further include sizing the alpha calciumsulfate hemihydrate to a median particle size from about 1 micron toabout 10 micron, prior to combining the alpha calcium sulfatehemihydrate and water to form a slurry. For example, the alpha calciumsulfate hemihydrate may be sized by jet milling or other suitable sizingor grinding processes known to those in the art.

In certain embodiments, the slurry is autoclaved to form alpha calciumsulfate hemihydrate whiskers in water. In one embodiment, the step ofautoclaving the slurry includes subjecting the slurry to saturated steamat a pressure from about 0 psig to about 50 psig and a temperature fromabout 100° C. to about 150° C. for a duration from about 30 minutes toabout 8 hours. For example, the step of autoclaving the slurry mayinclude subjecting the slurry to saturated steam at a pressure fromabout 1 psig to about 30 psig and a temperature from about 101° C. toabout 134° C. for a duration from about 30 minutes to about 8 hours. Forexample, such autoclave parameters may be applied in a small-scaleproduction setting, such as in the lab. For example, the step ofautoclaving the slurry may include subjecting the slurry to saturatedsteam at a pressure from about 30 psig to about 52 psig and atemperature from about 134° C. to about 150° C. for a duration fromabout 30 minutes to about 8 hours. For example, such autoclaveparameters may be applied in an industrial-scale production setting.Without being bound by a particular theory, it is believed that thecalcium sulfate dihydrate crystals dissolve during the autoclave processand reform as hemihydrate whiskers at the high pressure.

The alpha calcium sulfate hemihydrate whiskers may then be dewatered,i.e., the whiskers may be separated from water. In certain embodiments,the step of dewatering the alpha calcium sulfate hemihydrate whiskersincludes filtering, vacuuming, centrifuging, or a combination thereof.For example, a screen filter may be used to dewater the whiskers. In oneembodiment, the step of dewatering the alpha calcium sulfate hemihydratewhiskers includes filtering the alpha calcium sulfate hemihydratewhiskers

The alpha calcium sulfate hemihydrate whiskers may then be heated, or“dead burned,” to achieve a stable, insoluble anhydrite form. In certainembodiments, the step of heating the alpha calcium sulfate hemihydratewhiskers to form anhydrite calcium sulfate whiskers includes heating thealpha calcium sulfate hemihydrate whiskers at a temperature from about500° C. to about 900° C. for a duration from about 1 to about 24 hours.

Anhydrite calcium sulfate whiskers may be produced by the methodsdisclosed herein. In certain embodiments, the whiskers have a Mohshardness from about 3 to about 3.5. In certain embodiments, the whiskersare thermally stable up to at least 1450° C. In certain embodiments, thewhiskers have a mean aspect ratio of at least 30. As used herein, theterm “aspect ratio” refers to the ratio of the length of a calciumsulfate whisker to its diameter, and the “mean aspect ratio” is theratio of the average whisker length to the average whisker diameter fora plurality of whiskers.

Composites and Methods of Making

In certain embodiments, composites including whiskers are also provided.For example, the whiskers described herein may be lightweight and havedesirable properties for structural reinforcement, and thermal andacoustic insulation. For example, the whiskers may provide improveddimensional and thermal stability, increased strength and toughness, andhigher fluidity (e.g., for improved mold casting). Moreover, thewhiskers may provide improved surface quality and aesthetics forcomposites, because they are typically finer and smoother than fibers.

In certain embodiments, methods of making composites include combiningalpha-derived anhydrite calcium sulfate whiskers with a base material toform a composite. In certain embodiments, composites includealpha-derived anhydrite calcium sulfate whiskers and a base material.

For example, the base material may include a material selected from thegroup consisting of paints, coatings, paper, paper precursors, films,plastics, resins, gypsum wallboard, facers (including fiberglass mat),cement and concrete, and combinations thereof. Generally, alpha-derivedanhydrite calcium sulfate whiskers may be used in any applications wheretraditional fillers, opacification agents, or reinforcing additives,such as but not limited to fiberglass, carbon black, calcium carbonate,silica, talc, kaolin, or aluminum hydroxide, are currently used. Forexample, alpha-derived anhydrite calcium sulfate whiskers may becombined with paper materials, plastics, including elastomers,thermoplastics, and thermosets, and/or resins, including epoxies, andthermoset resins. In certain embodiments, the base material ispolyethylene, polypropylene, polybutylene, polyester, polyethyleneterephthalate (PET), nylon 6, nylon 66, polyurethane, epoxy, nitrilerubber, butyl rubber, silicone rubber, PTFE, or a phenolic.

In one embodiment, the step of combining the alpha-derived anhydritecalcium sulfate whiskers and the base material includes combining thealpha-derived anhydrite calcium sulfate whiskers with the base materialsuch that the alpha-derived anhydrite calcium sulfate whiskers arepresent in the composite in an amount from about 1 to about 50 percentby weight of the composite, such as from about 3 to about 40 percent byweight of the composite, or from about 3 to about 10 percent by weightof the composite. In one embodiment, a composite includes from about 3to about 10 percent by weight alpha-derived anhydrite calcium sulfatewhiskers.

In one embodiment of a composite, the base material includespolypropylene, the composite includes from about 2 to about 20 percentby weight alpha-derived anhydrite calcium sulfate whiskers, such as fromabout 4 to about 10 percent by weight alpha-derived anhydrite calciumsulfate whiskers, and the composite has an elastic modulus of at least1000 MPa.

The mechanical properties of whisker-reinforced polymer composites maybe further improved by a proper treatment of the whisker surface beforethey are introduced into the base material. For example, silane couplingagents, or other additives capable of coupling or compatibilizinginorganic materials to organic resins, such as3-methacryloxypropyltrimethoxysilane, may be used.

EXAMPLES

Embodiments of the calcium sulfate whiskers and composites disclosedherein were manufactured and tested. The results are shown below and atFIGS. 1-3.

Example 1

Calcium sulfate whiskers were prepared using alpha and beta hemihydrateparticles mined from the same location, according to the followingmethod. Calcium sulfate hemihydrate was combined with water to form aslurry containing 5 weight percent calcium sulfate. The slurry wasautoclaved at a pressure of 10 psig and a temperature of 115° C. for 2hours to form whiskers. The resulting whiskers were dewatered by screenfiltering for 5 minutes. The dewatered whiskers were heated at atemperature of 600° C. for 3 hours to form stable, insoluble calciumsulfate anhydrite whiskers. The resulting alpha and beta calcium sulfateanhydrite whiskers were then photographed.

FIG. 1 is a micrograph (600×400 microns) taken by optical microscopyshowing the alpha-derived anhydrite calcium sulfate whiskers preparedaccording to the above method. FIG. 2 is a micrograph (600×400 microns)taken by optical microscopy showing the beta-derived anhydrite calciumsulfate whiskers prepared by the same method as the alpha whiskers.These micrographs show the distinct morphology (e.g., length, diameter)of the alpha versus beta whiskers. For example, the alpha whiskers aremuch longer than the beta whiskers, and have a greater aspect ratio thanthe beta whiskers. Furthermore, the micrographs reveal that the growthrate is much faster for alpha whiskers than beta whiskers.

Specifically, it has been determined that the alpha calcium sulfate hasa crystal growth rate of at least 3-5 times that of beta calciumsulfate. Thus, the autoclave time needed to make whiskers of identicallength is much shorter with the alpha calcium sulfate, reducing thenecessary processing time. This is significant because the low growthrate of beta calcium sulfate whiskers led to high economical costs formanufacturing, making the practical applications of calcium sulfatewhiskers cost-prohibitive. Without being bound by a particular theory,it is believe that due to its “pre-autoclaved” nature, alpha calciumsulfate is able to more quickly grow into elongated whiskers under theautoclave environment.

Example 2

Alpha and beta calcium sulfate whiskers were prepared in accordance withthe method of Example 1. The whiskers were combined with polypropyleneto form composites and the stiffness/flexural modulus of each compositewas measured according to the Chord method and compared against thestiffness/flexural modulus of the polypropylene alone. The results areshown below in Table 1.

The whiskers were either premixed or side fed with the polypropylene.Premixed refers to the whiskers being combined with extrudedpolypropylene pellets prior to melting to form a composite. Side fedrefers to the whiskers being fed into the extruder when thepolypropylene is already molten.

TABLE 1 Stiffness/Flexural Modulus of Whisker- Reinforced PolypropyleneComposites Flexural Modulus of Flexural Whisker-Reinforced Modulus ofIncreased Polypropylene Polypropylene Stiffness Sample (MPa) (MPa) (%)Beta-derived 813 863 −5.8 whiskers: 10 wt. % premixed Beta-derived 11061085 1.9 whiskers: 10 wt. % premixed Alpha-derived 1060 754 40.6%whiskers: 10.1 wt. % premixed Alpha-derived 1240 754 64.5% whiskers:4.39 wt. % side fed Alpha-derived 1630 834 95.4% whiskers: 9.25 wt. %side fed Alpha-derived 1886 849 122.1% whiskers: 9.49 wt. % side fed

These results show that polypropylene composites with alpha whiskerreinforcement display much better mechanical properties than compositeswith comparable beta whisker reinforcement. Without being bound by aparticular theory, it is believed that the premixed process causes thewhiskers to be ground up by the unmelted polypropylene pellets, whichmay result in a lessened reinforcing effect of the whiskers. However,the premixed alpha whiskers performed surprisingly well, showing a 40%increase in stiffness, as compared to the premixed beta whiskers, whichshowed negligible stiffness increases.

FIG. 3 shows the elastic modulus of polypropylene versus a composite ofpolypropylene and 4.39 wt. % alpha calcium sulfate whiskers, measuredaccording to the Flexural Chord, Flexural Tangent, and Flexural Secantmethods. As shown, the stiffness of the polypropylene compositeincreases significantly (up to 65%) with the addition of the alphawhiskers.

Example 3

The commercially obtained alpha calcium sulfate particles used in theforegoing examples had a median particle size of 15.4 micron and amaximum particle size of 120 micron. Micronized alpha calcium sulfateparticles were also prepared and tested. Specifically, commerciallyobtained alpha calcium sulfate particles were ground in a jet mill to amedian particle size of 3.06 micron and a maximum particle size of about12 micron. Thus, the micronized particles were smaller and more uniformin size than the commercially obtained calcium sulfate particles, asshown in Table 2.

TABLE 2 Micronized Alpha Calcium Sulfate Particle DimensionDistributions d₁₀ Particle D₅₀ Particle D₉₀ Particle d₁₀₀ Particle Size:10^(th) Size: 50^(th) Size: 90^(th) Size: 100^(th) Percentile PercentilePercentile Percentile Time (μm) (μm) (μm) (μm) Before 2.55 15.72 52.83120.22 Micronizing After 1.13 3.06 6.42 12.00 Micronizing

The particles were jet milled on a fluid energy mill employingcompressed air to produce the smaller particles. High-speed rotationwithin the mill subjects the calcium sulfate particles toparticle-on-particle impact. As shown in Table 2, the micronizedparticles displayed a maximum size 10 times smaller than thecommercially obtained particles. Also, the micronized particles had amaximum particle size less than 4 times the average particle size.

Calcium sulfate whiskers were prepared by the method of Example 1, usingthe micronized alpha particles. Whiskers were also prepared using thecommercially obtained alpha particles and the micronized alphaparticles, but at a lower concentration of 2.5 wt. % in the slurry.Comparative samples of alpha whiskers made using commercially obtainedcalcium sulfate were made at a concentration of 5 wt. % in the slurry.Commercially available magnesium oxysulfate whiskers were also obtainedfor comparative purposes. The average length and diameter was measuredby scanning electron microscopy for the resulting whiskers, and theaspect ratio was calculated. The results are shown in Table 3.

TABLE 3 Length, Diameter, and Aspect Ratio for Various Whisker SamplesAverage Length Average Diameter Aspect Ratio Sample (μm) (μm) (L/D) LowConcentration 71.1 1.27 56.1 Whiskers Standard Alpha 64.4 1.36 47.5Whiskers (Example 1) Micronized Alpha 48.8 0.75 64.9 Whiskers MicronizedAlpha 52.6 0.48 109 and Low Concentration Whiskers

As shown in Table 3, the micronized alpha whiskers surprisingly had asignificantly higher aspect ratio than the standard alpha whiskers(almost 40% greater), with a slightly shorter length and a smallerdiameter. The low concentration commercial particle-derived whiskersalso showed a slightly higher aspect ratio than the standard whiskers,while the low concentration micronized particle-derived whiskers showeda significantly higher aspect ratio than all other samples. Themagnesium sulfate whiskers are much shorter than the calcium sulfatewhiskers, and have an even smaller diameter. Generally, an aspect ratioabove 30 is desirable for reinforcement applications, with higher aspectratios providing better structural reinforcement. Also, finer whiskerstend not to settle in the dispersion after autoclaving, such that theycan avoid the crowdedness that often results in localized excessivelyhigh concentration of crystals and thus short whiskers. However,micronizing the calcium sulfate adds processing time and cost to thewhisker manufacturing process. Overall, these examples show that alphacalcium sulfate whiskers can be manufactured to haveapplication-specific dimensions.

Embodiments of the present disclosure further include any one or more ofthe following paragraphs:

1. A method of making anhydrite calcium sulfate whiskers, comprising:

combining alpha calcium sulfate hemihydrate and water to form a slurry;

autoclaving the slurry to form alpha calcium sulfate hemihydratewhiskers in water;

dewatering the alpha calcium sulfate hemihydrate whiskers; and

heating the alpha calcium sulfate hemihydrate whiskers to form anhydritecalcium sulfate whiskers.

2. The method of paragraph 1, wherein the step of combining the alphacalcium sulfate hemihydrate and water comprises combining the alphacalcium sulfate hemihydrate with the water such that the alpha calciumsulfate is present in the slurry in an amount from about 1 to about 30percent by weight of the slurry.

3. The method of paragraph 1, wherein the step of combining the alphacalcium sulfate hemihydrate and water comprises combining the alphacalcium sulfate hemihydrate with the water such that the alpha calciumsulfate is present in the slurry in an amount from about 2 to about 15percent by weight of the slurry.

4. The method of paragraphs 1 to 3, wherein the alpha calcium sulfatehemihydrate has a median particle size, D₅₀, from about 1 micron toabout 20 micron.

5. The method of paragraphs 1 to 3, further comprising sizing the alphacalcium sulfate hemihydrate to a median particle size, D₅₀, from about 1micron to about 10 micron, prior to combining the alpha calcium sulfatehemihydrate and water to form a slurry.

6. The method of paragraphs 1 to 5, wherein the step of autoclaving theslurry comprises subjecting the slurry to saturated steam at a pressurefrom about 5 psig to about 55 psig and a temperature from about 100° C.to about 150° C. for a duration from about 30 minutes to about 8 hours.

7. The method of paragraphs 1 to 6, wherein the step of dewatering thealpha calcium sulfate hemihydrate whiskers comprises filtering,vacuuming, centrifuging, or a combination thereof.

8. The method of paragraphs 1 to 6, wherein the step of dewatering thealpha calcium sulfate hemihydrate whiskers comprises filtering the alphacalcium sulfate hemihydrate whiskers for a duration from about 1 toabout 10 minutes.

9. The method of paragraphs 1 to 8, wherein the step of heating thealpha calcium sulfate hemihydrate whiskers to form anhydrite calciumsulfate whiskers comprises heating the alpha calcium sulfate hemihydratewhiskers at a temperature from about 500° C. to about 900° C. for aduration from about 1 to about 24 hours.

10. Anhydrite calcium sulfate whiskers made by the method of paragraphs1 to 9.

11. The anhydrite calcium sulfate whiskers of paragraph 10, wherein thewhiskers have a Mohs hardness from about 3 to about 3.5.

12. The anhydrite calcium sulfate whiskers of paragraph 10 or 11,wherein the whiskers are thermally stable up to at least 1450° C.

13. The anhydrite calcium sulfate whiskers of paragraphs 10 to 12,wherein the whiskers have a mean aspect ratio of at least 30.

14. A method of making a composite, comprising combining alpha-derivedanhydrite calcium sulfate whiskers with a base material to form acomposite.

15. The method of paragraph 14, wherein the base material comprises amaterial selected from the group consisting of paints, coatings, paper,paper precursors, films, plastics, resins, gypsum wallboard, facers,cement and concrete, and combinations thereof.

16. The method of paragraph 14, wherein the base material is selectedfrom the group consisting of polyethylene, polypropylene, polybutylene,polyester, polyethylene terephthalate (PET), nylon 6, nylon 66,polyurethane, epoxy, nitrile rubber, butyl rubber, silicone rubber,PTFE, phenolics, and combinations thereof.

17. The method of paragraphs 14 to 16, wherein the step of combining thealpha-derived anhydrite calcium sulfate whiskers and the base materialcomprises combining the alpha-derived anhydrite calcium sulfate whiskerswith the base material such that the alpha-derived anhydrite calciumsulfate whiskers are present in the composite in an amount from about 1to about 50 percent by weight of the composite.

18. A composite, comprising alpha-derived anhydrite calcium sulfatewhiskers and a base material.

19. The composite of paragraph 18, wherein the base material comprises amaterial selected from the group consisting of paints, coatings, paper,paper precursors, films, plastics, resins, gypsum wallboard, facers,cement and concrete, and combinations thereof.

20. The composite of paragraph 18 or 19, comprising from about 1 toabout 50 percent by weight alpha-derived anhydrite calcium sulfatewhiskers.

21. The composite of paragraph 18, wherein:

the base material comprises polypropylene;

the composite comprises from about 2 to about 20 percent by weightalpha-derived anhydrite calcium sulfate whiskers; and

the composite has an elastic modulus of at least 1000 MPa.

22. The composite of paragraphs 18 to 21, wherein the alpha-derivedanhydrite calcium sulfate whiskers have a mean aspect ratio of at least30.

23. The composite of paragraph 18, 19, 20 or 22, wherein the basematerial is selected from the group consisting of thermoset plastics,thermoplastics, and combinations thereof.

24. The composite of paragraph 18, 19, 20, or 22, wherein the basematerial is selected from the group consisting of polyethylene,polypropylene, polybutylene, polyester, polyethylene terephthalate(PET), nylon 6, nylon 66, polyurethane, epoxy, nitrile rubber, butylrubber, silicone rubber, PTFE, phenolics, and combinations thereof.

While the disclosure has been described with reference to a number ofembodiments, it will be understood by those skilled in the art that thedisclosure is not limited to such disclosed embodiments. Rather, thedisclosed embodiments can be modified to incorporate any number ofvariations, alterations, substitutions, or equivalent arrangements notdescribed herein, but which are commensurate with the spirit and scopeof the disclosure. Additionally, while various features of the inventionhave been described, it is to be understood that aspects of theinvention may include only some of the described features. Moreover,while features of the embodiments have been discussed separately, itshould be understood that any of the features disclosed herein may becombined in composites, whiskers, and methods for making the same, withor without any other of the features or combinations of features.Accordingly, the invention is not to be seen as limited by the foregoingdescription, but is only limited by the scope of the appended claims.

What is claimed is:
 1. A method of making anhydrite calcium sulfatewhiskers, comprising: combining alpha calcium sulfate hemihydrate andwater to form a slurry; autoclaving the slurry to form alpha calciumsulfate hemihydrate whiskers in water; dewatering the alpha calciumsulfate hemihydrate whiskers; and heating the alpha calcium sulfatehemihydrate whiskers to form anhydrite calcium sulfate whiskers.
 2. Themethod of claim 1, wherein the step of combining the alpha calciumsulfate hemihydrate and water comprises combining the alpha calciumsulfate hemihydrate with the water such that the alpha calcium sulfateis present in the slurry in an amount from about 1 to about 30 percentby weight of the slurry.
 3. The method of claim 1, wherein the step ofcombining the alpha calcium sulfate hemihydrate and water comprisescombining the alpha calcium sulfate hemihydrate with the water such thatthe alpha calcium sulfate is present in the slurry in an amount fromabout 2 to about 15 percent by weight of the slurry.
 4. The method ofclaim 1, wherein the alpha calcium sulfate hemihydrate has a medianparticle size, D₅₀, from about 1 micron to about 20 micron.
 5. Themethod of claim 1, further comprising sizing the alpha calcium sulfatehemihydrate to a median particle size, D₅₀, from about 1 micron to about10 micron, prior to combining the alpha calcium sulfate hemihydrate andwater to form a slurry.
 6. The method of claim 1, wherein the step ofautoclaving the slurry comprises subjecting the slurry to saturatedsteam at a pressure from about 5 psig to about 55 psig and a temperaturefrom about 100° C. to about 150° C. for a duration from about 30 minutesto about 8 hours.
 7. The method of claim 1, wherein the step of heatingthe alpha calcium sulfate hemihydrate whiskers to form anhydrite calciumsulfate whiskers comprises heating the alpha calcium sulfate hemihydratewhiskers at a temperature from about 500° C. to about 900° C. for aduration from about 1 to about 24 hours.
 8. Anhydrite calcium sulfatewhiskers made by the method of claim
 1. 9. The anhydrite calcium sulfatewhiskers of claim 8, wherein the whiskers have a Mohs hardness fromabout 3 to about 3.5.
 10. The anhydrite calcium sulfate whiskers ofclaim 8, wherein the whiskers are thermally stable up to at least 1450°C.
 11. The anhydrite calcium sulfate whiskers of claim 8, wherein thewhiskers have a mean aspect ratio of at least
 30. 12. A method of makinga composite, comprising combining alpha-derived anhydrite calciumsulfate whiskers with a base material to form a composite.
 13. Themethod of claim 12, wherein the base material comprises a materialselected from the group consisting of paints, coatings, paper, paperprecursors, films, plastics, resins, gypsum wallboard, facers, cementand concrete, and combinations thereof.
 14. The method of claim 12,wherein the base material is selected from the group consisting ofpolyethylene, polypropylene, polybutylene, polyester, polyethyleneterephthalate (PET), nylon 6, nylon 66, polyurethane, epoxy, nitrilerubber, butyl rubber, silicone rubber, PTFE, phenolics, and combinationsthereof.
 15. The method of claim 12, wherein the step of combining thealpha-derived anhydrite calcium sulfate whiskers and the base materialcomprises combining the alpha-derived anhydrite calcium sulfate whiskerswith the base material such that the alpha-derived anhydrite calciumsulfate whiskers are present in the composite in an amount from about 1to about 50 percent by weight of the composite.
 16. A composite,comprising alpha-derived anhydrite calcium sulfate whiskers and a basematerial.
 17. The composite of claim 16, wherein the base materialcomprises a material selected from the group consisting of paints,coatings, paper, paper precursors, films, plastics, resins, gypsumwallboard, facers, cement and concrete, and combinations thereof. 18.The composite of claim 16, comprising from about 1 to about 50 percentby weight alpha-derived anhydrite calcium sulfate whiskers.
 19. Thecomposite of claim 16, wherein: the base material comprisespolypropylene; the composite comprises from about 2 to about 20 percentby weight alpha-derived anhydrite calcium sulfate whiskers; and thecomposite has an elastic modulus of at least 1000 MPa.
 20. The compositeof claim 16, wherein the base material is selected from the groupconsisting of polyethylene, polypropylene, polybutylene, polyester,polyethylene terephthalate (PET), nylon 6, nylon 66, polyurethane,epoxy, nitrile rubber, butyl rubber, silicone rubber, PTFE, phenolics,and combinations thereof.